As interlayer insulating films in semiconductor elements or the like, silicon oxide films (SiO2 films) formed by a vacuum process such as CVD (chemical vapor deposition) process have been heretofore used frequently, but these SiO2 films have high dielectric constant of about 4.
In recent years, lowering of dielectric constant of the interlayer insulating films for the purposes of fining VLSI elements and saving power consumption has been paid attention. Such insulating film materials of low dielectric constant exhibit dielectric constant of about 2.2 to 3.5, and they are expected to contribute to enhancement of performance of semiconductor elements. However, they have lower mechanical strength than the heretofore known silicon oxide films formed by a vacuum process and have an elastic modulus, as measured by nanoindentention method, of about 1 to 2.0 GMPa.
By the way, in order to form Damascene interconnect, a process comprising depositing a metal that becomes a interconnect material on an insulating layer having a trench at the place where a interconnect should be arranged and then removing an excess metal material by chemical mechanical polishing is usually employed. For the purpose of preventing metal atoms (e.g., copper atoms) constituting the metal material that is a interconnect material from diffusing into the insulating layer, a barrier metal layer is formed between the insulating layer and the metal layer that becomes a interconnect material (see, for example, patent document 1). As a material of the barrier metal layer, a metal having high hardness, such as tantalum, tantalum nitride, titanium or titanium nitride, is usually employed. In the case where such a barrier metal layer is formed, an excess metal material is removed by chemical mechanical polishing and then the barrier metal layer is further removed, whereby Damascene interconnect is formed.
If an insulating film material of a low dielectric constant and a barrier metal are both adopted in order to enhance performance of a semiconductor element or the like, the insulating film material of a low dielectric constant and low mechanical strength that is present in the underlying layer suffers damage such as occurrence of a large number of scratches (scratchy surface defects) or excessive polishing, resulting in a problem that no favorable Damascene interconnect is obtained.
In order to solve such a problem, it has been proposed to form a stronger insulating layer between the insulating film of a low dielectric constant and the barrier metal layer to reduce damage of the insulating layer of a low dielectric constant (see, for example, patent document 2). The stronger insulating layer formed for this purpose is referred to as a “cap layer”.
In the case where the cap layer is formed as above, the excess metal material and the barrier metal layer are removed by chemical mechanical polishing and then the cap layer is further removed in the chemical mechanical polishing process in the production of a semiconductor element, whereby Damascene interconnect is formed.
Since removal of the excess metal material takes a longest time, this removal process is sometimes separated and carried out as a different process to increase throughput. In such a case, after the excess metal material is removed, removal of the barrier metal layer and removal of the cap layer are carried out as one process or carried out separately as different processes.
In the former case, there is required a chemical mechanical polishing process wherein the barrier metal layer and the cap layer can be efficiently removed by polishing and damage to the insulating film material of a low dielectric constant present in the underlying layer is reduced. In the latter case, there is required a chemical mechanical polishing process wherein the cap layer can be efficiently removed by polishing and damage to the insulating film material of a low dielectric constant present in the underlying layer is reduced.
In order to meet the above requirements, it has been proposed to use colloidal silica having uniform particle diameters as a chemical mechanical polishing aqueous dispersion for use in the chemical mechanical polishing process to remove the barrier metal layer or the cap layer (see, for example, patent document 3). In this method, however, removal rates for the barrier metal, the cap layer and the insulating layer of a low dielectric constant are badly balanced, so that the insulating layer of a low dielectric constant is excessively polished, and as a result, many scratches sometimes occur on the insulating layer.
Further, a method of using a pad of low hardness as a polishing pad and a method of polishing under the conditions of low head pressure in the polishing process have been studied. In these methods, however, the removal rate for the barrier metal is insufficient.
As described above, a chemical mechanical polishing process wherein a barrier metal layer and a cap layer can be efficiently removed by polishing and damage to an insulating film material of a low dielectric constant present in the underlying layer is reduced, and a chemical mechanical polishing aqueous dispersion used for the process have not been proposed yet.
Patent document 1: Japanese Patent Laid-Open Publication No. 139092/1996
Patent document 2: Japanese Patent Laid-Open Publication No. 77920/2003
Patent document 3: Japanese Patent Laid-Open Publication No. 270545/2002